1. Field of the Invention
The present invention relates to a hydrodynamic bearing device that uses a dynamic pressure bearing, a spindle motor including the hydrodynamic bearing device, and a method for manufacturing the hydrodynamic bearing device.
2. Description of the Related Art
In place of a ball bearing device conventionally used, a hydrodynamic bearing device that excels in rotation precision and that excels in sound silence in comparison with ball bearing is being widely used as a bearing device used in a spindle motor of a hard disc, a polygon mirror, an optical disc device or the like.
In the hard disc or the like that uses such hydrodynamic bearing device, improvement in recording density or enhancement in long-term reliability is advancing, and accompanied therewith, cleanliness demanded for the components is becoming extremely strict. However, since the martensitic stainless which is the bearing member currently being used commonly is a magnetic material, dust is likely to attach thereto, and further improvement in cleanliness is becoming difficult. Thus, improving cleanliness by using an iron metal having an austenite structure such as austenite stainless or austenite high strength steel which is a non-magnetic body is being reviewed.
However, although the conventional martensitic stainless obtains a hardness of about HRc (Rockwell hardness) 56 by hardening, the iron metal having austenite structure has a hardness of about HRc 10 to 25 as it cannot be hardened, and thus when used in such state, the abrasion of the bearing surface advances. When the bearing surface is abraded, rotation abnormality or, in the worst case, a lock of the bearing occurs, thereby greatly lowering the reliability of the hydrodynamic bearing device, and thus the austenite stainless cannot be used as it is.
In order to address the above problems, in the hydrodynamic bearing device disclosed in JP-A 2002-188638, a nitriding process is performed on a shaft made of austenite stainless steel to improve the surface hardness. For instance, after making the SUS 303, which is an austenite stainless steel, to a required shape through cutting, a dynamic pressure groove is formed through plastic forming. Subsequently, finish machining is performed by polishing, and surface effect process is performed by nitriding process of (1) salt bath nitriding, (2) ion nitriding, (3) gas nitrocarburizing and the like to fabricate the shaft.
In a slide bearing device disclosed in JP-A 2002-357223, an excellent abrasion resistance property is obtained by forming a DLC (Diamond Like Carbon) coating on the bearing surface.
Further, in JP-A 2002-161371, the lubricating property is enhanced by spraying injection fine particle in which the fine particles of a metal constituting the parent phase and the fine particles of a solid lubricant are mixed, and forming a coated layer in which the solid lubricant is dispersed in the parent phase. For example, the fine particles are sprayed on the shaft with collar made of heat-resistant stainless steel (SUS 304) used in HDD (Hard Disc Drive) spindle motor to form the lubricating coated layer.
However, in the hydrodynamic bearing device of the conventional configuration disclosed in JP-A 2002-188638, surface roughness involved in nitriding process occurs, and when used directly after the nitriding process, problems such as mixture (contamination) due to cleaning failure, or advancement of abrasion of the bearing surface arise. Further, since the nitriding process requires a vacuum device, the manufacturing cost increases.
In the slide bearing device of the conventional configuration disclosed in JP-A 2002-357223, as the stripping strength of the DLC is low, the DLC is stripped when a momentary high stress is applied thereby causing the bearing surfaces to contact each other by way of the DLC and locking the bearing. Further, since the nitriding process requires a vacuum device, similar to above, the manufacturing cost increases.
In the lubricating coated layer of the conventional configuration disclosed in JP-A 2002-161371, as the metal constituting the parent phase is a soft metal, the abrasion of the surface easily advances. Further, since the fine particles of soft metal constituting the parent phase and the fine particles of the solid lubricating material are mixed and sprayed, in the last half of spraying in which the surface is formed, the solid lubricating material is sprayed on the soft metal layer, and thus the surface roughness roughens. As described in the patent document, in the hydrodynamic bearing in which the operating fluid is gas, micro-dimples are sometimes effective, but in the hydrodynamic bearing in which the operating fluid is liquid, mixture (contamination) problem arises due to cleaning failure or problem of advancement of abrasion of the bearing surface.